One cannot usually control the power level (~600 mW maximum) or frequency (~900
MHz or ~1800 MHz) of a mobile phone without special equipment as those
parameters are set by the mobile network base station the phone is
connected to. In this case, the phone was operating in the US ~900 MHz
band. The indicated signal strength was low (two of five bars) so the phone
should have been near, but not necessarily at, its maximum power.
Figure 3 shows a side view of the test setup. The phone was placed over
the path crossing the ground break so as to maximize the EMI generated
in the path. Remember Murphy's Law! If the phone protective cover had
been removed to allow closer spacing to the board, the induced EMI
would have been larger than that observed.
Side View of Test Setup
Figure 4 shows the mobile phone next to the board to get a clear view of both the phone and board.
Figure 4. Mobile Phone and Test Board
Figure 5 shows the resulting scope plot. The period of the sine wave
was a little over one nanosecond with a peak amplitude of over two
Volts! An amplitude of two Volts peak into the 50 Ohm termination in
the scope is certainly enough to cause effects on real circuits. There
is about 40 mW of power delivered to the scope and also to the 47 Ohm
termination, a total of about 80 mW. This is a significant fraction of
the phone's transmitting power.
Figure 5. Signal Induced in Board by Mobile Phone
(Vertical = 1 Volt/div, Horizontal = 1 ns/div)
Mobile phones are not high powered devices, but they can be very close
to some electronic products. When they are very close, as in this
example, significant EMI can be generated in the nearby electronic
device. There are many cases where mobile phones can be in physical
contact with other small electronic devices, such as in a pocket or